Driving LEDs in LCD backlight

ABSTRACT

A backlight driver includes current sources that are connected between LED strings and a number of bias voltages. There can be any number of different bias voltages, each at a ground potential or higher voltage. The bias voltage is selected for a particular LED string in order to reduce a current drop across the current source. This reduces the power consumption of the current source and LED string. Heat dissipation is also reduced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 14/313,935, filed Jun. 24, 2014, issued as U.S. Pat. No.8,928,239 on Jan. 6, 2015, which is a continuation of U.S. patentapplication Ser. No. 13/607,413, filed Sep. 7, 2012, issued as U.S. Pat.No. 8,760,068 on Jun. 24, 2014, which claims the benefit of U.S.provisional application 61/532,085, filed Sep. 7, 2011. Theseapplications are incorporated by reference along with all otherreferences cited in this application.

BACKGROUND OF THE INVENTION

This invention relates to the field of lighting, and more specificallyto backlighting for a visual displays.

Electronic visual displays including televisions, monitors, panels,screens, and the like are used to display computer output, television,video, and other visual media. Some visual display technologies, such asliquid crystal or liquid crystal display (LCD), use a backlight in orderto actively light the LCD panel display. LCDs are in use as computermonitors, televisions, tablet computers, projectors, smartphones,electronic picture frames, electronic signs, industrial equipmentdisplays, medical device displays, and many other visual display uses.

Despite the widespread success of existing backlighting technology,there is a need for further improvements. These include lowering thecost, improving the performance characteristics (e.g., colortemperature, white point, and others), reducing heat, and reducing powerconsumption.

Therefore, there is a need for improvements in the circuitry to drive abacklight of an LCD display.

BRIEF SUMMARY OF THE INVENTION

A backlight driver includes current sources that are connected betweenLED strings and a number of bias voltages. There can be any number ofdifferent bias voltages, each at a ground potential or higher voltage.The bias voltage is selected for a particular LED string in order toreduce a current drop across the current source. This reduces the powerconsumption of the current source and LED string. Heat dissipation isalso reduced.

A system selects the voltage of the negative terminal of the currentsource of each string depending on the voltage drop on the string.

In other implementations, instead of current sources, the system usesother means to control the current through the strings (e.g. resistors,voltage sources, or controls directly the current in the power supplyproviding power to each string).

An implementation provides a fully integrated solution in which thecircuitry resides on a single integrated circuit. However, in otherimplementations, the circuitry can be a discrete solutions or forsolutions that use multiple integrated circuits (or chips).

An implementation includes a multiple-output single-inductor buckconverter. However, other implementations includes other voltagegenerators including multiple bucks, linear regulators, current-sinkinglow-dropout regulators (LDOs), resistors, or other circuitry, andcombinations of these.

Other implementations use other power supplies available in the system(e.g., logic supplies) even if these supplies are not variable, or whenthe system uses internally generated but fixed supplies.

In other implementations, the polarity of the devices can be reversed orpartially reversed. For example, the current sources can be placed onthe anode (positive terminal) of the LED string instead of at thecathode (negative terminal).

Although various implementations directed to backlighting as discussed,aspects of the invention can be applied to circuits and systems notinvolving backlighting. The circuits and techniques can be applied towhite LEDs (WLEDs), but also to LEDs of different colors, notnecessarily white. LEDs come in many colors including red, orange,yellow, green, blue, violet, infrared, and many others. For example, asingle LED string can include LEDs of different colors. A system with abacklight driver of the invention can have multiple colors of LEDs(e.g., each string being a different color).

A positive voltage connected to an anode of the LED string can besupplied from an external source, internal source, boost converter,flyback, or other supply means.

In a specific implementation, a system includes: a first voltage input,second voltage input, and third voltage input; a first voltage generatorcircuit, connected to the first voltage input and generating a firstvoltage output, where the first voltage output is at a higher voltagelevel than the first voltage input; a first current source blockincluding a first current source node, connected to the second end ofthe first plurality of light emitting diodes; a second current sourceblock including a second current source node, connected to the fourthend of the first plurality of light emitting diodes; a firstmultiplexing block connected to the second and third voltage inputs, anda first multiplexing block output is connected to the first currentsource block; and a second multiplexing block connected to the secondand third voltage inputs, and a second multiplexing block output isconnected to the second current source block, where the voltage outputand the first and second current source nodes are configured to beconnected to a plurality of light emitting diodes.

A first set of light emitting diodes includes a first end connected tothe voltage output and a second end connected to the first currentsource node. A second set of light emitting diodes includes a third endcoupled to the voltage output and a fourth end connected to the secondcurrent source node.

In various implementations, the voltage generator circuit, first currentsource block, second current source block, first multiplexing block, andsecond multiplexing block reside on a single integrated circuitsubstrate. The voltage generator circuit, first current source block,second current source block, first multiplexing block, and secondmultiplexing block reside on a single integrated circuit substrate, andthe first and second plurality of light emitting diodes do not reside onthe same integrated circuit substrate where the voltage generatorcircuit, first current source block, second current source block, firstmultiplexing block, and second multiplexing block reside.

The system can include a first logic block connected to the first andsecond multiplexing blocks, where the first logic block controlsselection of the first and second multiplexing blocks. The voltagegenerator circuit can include a boost converter circuit. A secondvoltage generator circuit is connected to the first voltage input andgenerates at least a second voltage output and a third voltage output,where the first voltage output is connected to the second voltage input,and the second voltage output is connected to the third voltage input.The second voltage generator circuit can include a buck convertercircuit.

The system can include a second logic block including inputs connectedto the first current source node and the second current source node, anda second logic block output connected to the first voltage generatorcircuit. A third logic block includes an input connected to the secondlogic block output, and a first output of the third logic block isconnected to the second voltage input and a second output of the thirdlogic block output is connected to the third voltage input. The firstvoltage input can be VDD, the second voltage input can be ground, andthe second voltage is a voltage level between VDD and ground.

Some systems incorporating features of the invention can includetelevisions, monitors, panels, screens, flat panel display, LED panel,LCD panel, tablet computers, projectors, smartphones, electronic pictureframes, electronic signs, industrial equipment displays, medical devicedisplays, and many other visual displays.

In a specific implementation, A method includes: providing a first nodeto connect to a first set of light emitting diodes; providing a secondnode to connect to a second set of light emitting diodes; determining afirst voltage drop across the first set of light emitting diodes;determining a second voltage drop across the second set of lightemitting diodes; when the first voltage drop is greater than the secondvoltage drop, causing the first node to be at a ground voltage level andsecond node to be at a bias voltage level above ground; and when thesecond voltage drop is greater than the first voltage drop, causing thesecond node to be at a ground voltage level and first node to be at abias voltage level above ground.

The first set of light emitting diodes can include at least two lightemitting diodes coupled in series, as well as one or more light emittingdiodes in parallel. The method can include providing a buck circuit togenerate the bias voltage level. The method can include providing athird node to connect to the first and second sets of light emittingdiodes; providing a boost circuit to generate a stepped-up voltagelevel; and causing the third node to be at the stepped-up voltage level.

In a specific implementation, a method includes: providing a first nodeto connect to a first set of light emitting diodes; providing a secondnode to connect to a second set of light emitting diodes; providing athird node to connect to a third set of light emitting diodes;determining a first voltage drop across the first set of light emittingdiodes; determining a second voltage drop across the second set of lightemitting diodes; determining a third voltage drop across the third setof light emitting diodes; when the first voltage drop is greater thanthe second and third voltage drops, causing the first node to be at aground voltage level and second and third nodes to be at one or morevoltage levels above ground; and when the second voltage drop is greaterthan the first and third voltage drops, causing the second node to be atthe ground voltage level and first and third nodes to be at one or morevoltage levels above ground.

The method can include: when the third voltage drop is greater than thefirst and second voltage drops, causing the third node to be at theground voltage level and first and second nodes to be at one or morevoltage levels above ground. The method can include: providing firstcircuitry to determine the first, second, and third voltage drops; andproviding second circuitry to cause the first node to be at a groundvoltage level and to cause the second node to be at a ground voltagelevel, where the first and second circuitry reside on an integratedcircuit substrate that is separate from the first and second sets oflight emitting diodes.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand the accompanying drawings, in which like reference designationsrepresent like features throughout the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electronic visual display including a backlight circuit.

FIG. 2 shows a system having a backlight circuit and display panel.

FIG. 3 shows an edge-lit backlight.

FIG. 4 shows a full array backlight.

FIG. 5 shows a circuit schematic for a series configuration for an LEDstring.

FIG. 6 shows a circuit schematic for a parallel configuration for an LEDstring.

FIG. 7 shows a circuit schematic for a backlight driver.

FIG. 8 shows a circuit schematic for a backlight driver with differentsections identified.

FIG. 9 shows a block diagram of a backlight circuit.

FIG. 10 shows a logical circuit to determine a minimum voltage levelfrom a number on input nodes.

FIG. 11 shows a bias voltage generator circuit.

FIG. 12 shows a logical circuit to determine a minimum voltage levelconnected to a bias voltage generator circuit.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an electronic visual display 101 which circuitry of theinvention is used within. This patent application describes circuitry ofthe invention used within a backlight of a visual display. However thecircuitry and aspects of the invention can be used and applied to otherelectronic systems.

FIG. 2 shows a block diagram of an electronic system 202, such as avisual display, which includes an LCD panel 207 and backlight 215. Thelighting in this backlight includes light emitting diodes or LEDs. Inmany displays, the LCD panel is sandwiched together with the LEDbacklight. The LCD panel is on top or in front of the LED backlight. So,the light from the LED backlight shines through the LCD panel, lightingthe display.

LEDs are a semiconductor light source. When a light-emitting diode isforward biased (switched on), electrons are able to recombine withelectron holes within the device, releasing energy in the form ofphotons. This effect is called electroluminescence and the color of thelight (corresponding to the energy of the photon) is determined by theenergy gap of the semiconductor. LEDs are often small in area (less than1 square millimeter), and integrated optical components may be used toshape its radiation pattern.

Although a backlight driver is described with respect to LED technologyin this patent application, the principles and aspects of the inventioncan be applied to other lighting technologies. For example, instead ofstrings of LEDs, the circuitry presented can be used with strings ofother lighting technologies.

FIG. 3 shows an LED backlight with strings of LEDs 303 and 305 at twoopposing edges of the backlight. This configuration may be called anedge-lit backlight. Each string has a plurality of LEDs 308. Thebacklight may include a light well or diffuser, or both, to distributethe light evenly.

The figure shows a string arranged in line. However, a string of LEDscan have any configures or positioned to have any desired orientation,such as circular, box, or other.

FIG. 4 shows an LED backlight having an array of LED strings 404. A fullarray backlight can have any number of LED strings, and depends on thesize of the display and desired brightness.

FIG. 5 shows a circuit schematic of an LED string 505 with a pluralityof LEDs 509, which are represented by diode symbols, connected in aseries.

FIG. 6 shows a circuit schematic of an LED string 606 with LEDs 613 and616 connected in parallel. An LED string can include any number of LEDsin series or parallel, or any combination of series and parallelconnection.

FIG. 7 shows a circuit schematic of a backlight driver. The circuitryincludes a number of LED strings, which are connected between a positiverail and current sources. More details are provided below.

FIG. 8 shows the circuitry of FIG. 7 with boxes identifying sections ofthe circuitry. An integrated circuitry boundary 808 is indicated. Thecircuitry within the boundaries of box 808 is fabricated on a singleintegrated circuit.

The backlight driver circuit includes a boost or step-up converter 812.A boost converter is a power converter to obtain an output DC voltagegreater than its input DC voltage. The boost converter output isconnected to a first end or positive end (e.g., anode for n-channel typeLEDs) of the LED strings.

On a second end or negative end (e.g., cathode) of the LED strings, eachLED strings is connected to a current source 816. Each current source isselectively connected to a voltage potential, ground, V1, or V2, orothers. The voltages for the cathode (e.g., V1 or V2) can be generatedusing a buck converter 819. A buck converter is similar to a boostconverter, but is a step-down DC-to-DC converter. The output DC voltageof a buck converter is less than its input DC voltage.

A logic and adaptive supply selection circuitry block 824 selects whichvoltage (e.g., ground, V1, or V2) to connect to a current source. In animplementation, a voltage to be connected to the current source issufficiently low to keep the LEDs forward biased and on (or lighted),but sufficiently high (e.g., a voltage above ground) in order to reducethe voltage drop across the current source. This will reduce powerconsumption of the current source, and the string to which it isconnected in series. Heat is also reduced.

Logic 824 is connected to the cathode nodes of each LED string. Logic824 determines the minimum voltage at the cathode nodes. This stringhaving the minimum voltage at its cathode will be the voltage having thegreatest voltage drop across the string. The output of logic 824 isconnected to the buck converter to adjust voltages which are connectedto the current sources (e.g., ground, V1, or V2). This reduces currentconsumption of the strings.

In an implementation, the positive end of the LED strings (generated bythe boost converter circuit) is connected to from about 40 volts toabout 60 volts. There are typically from about 1 to about 16 LEDstrings.

FIG. 9 shows a block diagram of backlight circuitry. An anode end 909 ofLED strings 914 is connected to a voltage supply 918 (outputting avoltage VP). The voltage supply can be from an on-chip voltage supply,external voltage supply, boost converter circuit, or other. A cathodeend 922 of the LEDs strings is connected to individual current sources926. An end 931 of the current source is connected to an output of amultiplexer 935. Inputs to the multiplexers can be from any number ofvoltage inputs including ground, V1, and V2. The number of voltages canbe labeled V1 to Vm, where m is an integer 1 or greater. Generally whenare n strings of LEDs, there will be m different voltages for input tothe multiplexers. The value of m will generally be less than n.

FIG. 10 shows logical circuitry block 1011 where inputs 1015 areconnected to nodes 922 of the circuitry in FIG. 9. An output 1020 isconnected to voltage supply 918 or a supply to generate voltages fornodes 931, or both.

In an implementation, the output of logical circuitry block 1011 gives aminimum voltage value by which an appropriate bias voltage Vbias (e.g.,ground, V1, V2, or other) is connected through multiplexers 935 to node922. This minimum voltage value is used to adjust the output of thevoltage going to node 931 of the current source 926. In particular, forthe LED string with lowest or minimum cathode voltage, this string hasthe greatest voltage drop across it. Then node 931 of the current sourcefor this string is connected to ground, while current sources connectedto other LED strings are connected to another higher voltage (e.g., biasvoltage Vbias). This reduces power consumption, while still allowing thediodes of the string to light.

In an implementation, the output of logical circuitry block 1011 gives aminimum voltage value by which voltage supply 918 is adjusted. Forexample, the voltage output VP can be adjusted to be less positive. Thisalso has the effect to reduce the power drop across the LED strings,thus lower power consumption.

In various implementations, output 1020 can be connected to control oradjust voltages at the anode or cathode of the strings, or both.

FIG. 11 shows a bias voltage generator circuit 1112 which takes an inputvoltage 1116 and generates one or more output voltages 1118. The outputvoltages are connected to V1, V2, V3, to Vm of multiplexer 935 of FIG.9. In an implementation, the voltage generator circuit is a buckconverter. In other implementations, the generator circuit is a boostconverter, linear voltage converter, or other voltage generatorcircuitry.

FIG. 12 shows a logical circuitry block 1011 as described for FIG. 10with an output (e.g., minimum voltage value) that is connected to avoltage generator circuit 1212. At its output 1218, voltage generatorcircuit 1212 generates voltages (e.g., V1, V2, to Vm) for the inputs tothe multiplexers 935.

LCD TV systems use backlight provided by white LEDs (WLEDs). The WLEDsare grouped in strings (series connection of several LEDs). Each stringcan contain up to about 30 WLEDs (typically 10-15) and a television cancontain between 3 to about 12 strings. Each string has a bias current oftypically from about 100 milliamps to about 300 milliamps and thiscurrent can be modulated using power width modulation (PWM). The typicalpower consumption of the WLED backlight is approximately 50 watts ormore.

A problem of this system is that each WLED has a different forwardvoltage. As an example, typical WLED forward voltage for a given kind ofLEDs can be 3.2 volts; however, each WLED of this kind can have forwardvoltages that ranges between 3.0 volts to 3.4 volts. String to stringmismatch can be as high as 10 volts (e.g., 0.4 volts*25 LEDs in series).This would lead to an excess power dissipation of 1 watt per string. Theexcess power is dissipated in the current source that provides the biasfor the LEDs.

To avoid (or limit) this issue, LEDs are “binned” for voltage. Binningrefers to the process of screening or measuring each LED one by one,usually by a tester machine, and sorting the LEDs into different binsdepending on their forward voltage and other performancecharacteristics. Typically this is done with 100 millivolts resolutionand allows having string-to-string voltage mismatches of less than 2.5volts. A drawback to binning that it is expensive, which leads toincreased system costs. Binning takes time and resources to accomplish.Some LEDs will not be used. Therefore, the selected LEDs (e.g., “bin 1”LEDs) having the desired performance characters will each cost more.

A 2.5-volt string-to-string mismatch is still very large and creates apower consumption that cannot be taken by an integrated circuit. Forthis reason, the current sources in the WLED systems are made usingexternal discrete devices (typically bipolar NPN transistors).

The circuitry in this patent can be used in a process to avoid binningof the LEDs and therefore reduce the cost of the system. Unbinned LEDscan be used in an LED backlight. The unbinned LEDs will cost less thanbinned LEDs, since an LED manufacturer's cost is reduced by shorteningor skipping the binning process.

This method can be used to drive current sources that are internal tothe integrated circuit and therefore the bill of materials (BOM) isreduced as well.

In an implementation of the backlight driver, the current sources are“floating” (i.e., not directly attached to ground) and can be attachedto different externally created voltages. In other implementations, thevoltages can be created internally, such as using circuitry on the sameintegrated circuit.

Referring to FIG. 8, containing “n” strings, the voltages on thecathodes (negative terminals) of each strings are read and fed to astate machine (or microcontroller), labeled as “LOGIC.” The LOGIC blockdetermines the string with the largest voltage drop, connects thenegative terminal of the current source of that string to ground, andadjusts the voltage of the boost converter in order to minimize thepower loss on that string.

Subsequently, the LOGIC adjusts the “m” voltages generated by anexternal multiple-outputs buck converter (or by any other means), tominimize the total power consumption (based on the voltage of all theremaining strings). This is performed using an algorithm. LOGIC attachesnegative terminal of each of the current sources of the remaining (n−1)strings to one of the m voltages generated as above. By appropriateselection of the external voltages, the typical total unwanted powerconsumption of the current sources can be reduced by more than 1/m.

This description of the invention has been presented for the purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise form described, and manymodifications and variations are possible in light of the teachingabove. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical applications.This description will enable others skilled in the art to best utilizeand practice the invention in various embodiments and with variousmodifications as are suited to a particular use. The scope of theinvention is defined by the following claims.

The invention claimed is:
 1. A system comprising: a first voltage input,second voltage input, and third voltage input; a first voltage generatorcircuit, coupled to the first voltage input and generating a firstvoltage output, wherein the first voltage output is at a higher voltagelevel than the first voltage input; a first source block comprising afirst source node, coupled to a first end of a first plurality ofserially-connected light emitting diodes; and a first multiplexing blockcoupled to the second and third voltage inputs, and a first multiplexingblock output is coupled to the first source block, wherein the firstvoltage output and the first source node are configured to be coupled toa second end of the first plurality of serially-connected light emittingdiodes.
 2. The system of claim 1 comprising: a second source blockcomprising a second source node, coupled to a first end of a secondplurality of serially-connected light emitting diodes; and a secondmultiplexing block coupled to the second and third voltage inputs, and asecond multiplexing block output is coupled to the second source block,wherein the first voltage output and the second source node areconfigured to be coupled to a second end of the second plurality ofserially-connected light emitting diodes.
 3. The system of claim 2wherein the first plurality of serially-connected light emitting diodesis coupled to the first voltage output, and the second plurality ofserially-connected light emitting diodes is coupled to the first voltageoutput.
 4. The system of claim 1 comprising a backlight for an displaypanel, wherein the backlight comprises the first plurality ofserially-connected light emitting diodes.
 5. The system of claim 1wherein the first voltage generator circuit, first current source block,second current source block, first multiplexing block, and secondmultiplexing block reside on a single integrated circuit substrate. 6.The system of claim 3 wherein the first voltage generator circuit, firstcurrent source block, second current source block, first multiplexingblock, and second multiplexing block reside on a single integratedcircuit substrate, and the first and second plurality ofserially-connected light emitting diodes do not reside on the sameintegrated circuit substrate where the first voltage generator circuit,first current source block, second current source block, firstmultiplexing block, and second multiplexing block reside.
 7. The systemof claim 1 wherein each light emitting diode in the first plurality ofserially-connected light emitting diodes illuminates a white color only.8. The system of claim 1 comprising: a first logic block coupled to thefirst multiplexing block, wherein the first logic block controlsselection of the first multiplexing block.
 9. The system of claim 8comprising: a second logic block comprising inputs coupled to the firstsource node, and a second logic block output coupled to the firstvoltage generator circuit.
 10. The system of claim 1 wherein the firstvoltage generator circuit comprises a boost converter circuit.
 11. Thesystem of claim 1 comprising: a second voltage generator circuit,coupled to the first voltage input and generating at least a secondvoltage output and a third voltage output, wherein the first voltageoutput is coupled to the second voltage input, and the second voltageoutput is coupled to the third voltage input.
 12. The system of claim 11wherein the second voltage generator circuit comprises a buck convertercircuit.
 13. A smartphone comprising the system of claim
 1. 14. Atelevision comprising the system of claim
 1. 15. A method comprising:providing a first voltage input, second voltage input, and third voltageinput; using a first voltage generator circuit, coupled to the firstvoltage input and generating a first voltage output, wherein the firstvoltage output is at a higher voltage level than the first voltageinput; coupling a first source block comprising a first source node to afirst end of a first plurality of light emitting diodes coupled inseries; and coupling a first multiplexing block to the second and thirdvoltage inputs, and a first multiplexing block output is coupled to thefirst source block, wherein the first voltage output and the firstsource node are configured to be coupled to a second end of the firstplurality of light emitting diodes coupled in series.
 16. The method ofclaim 15 comprising: coupling a second source block comprising a secondsource node to a first end of a second plurality of light emittingdiodes coupled in series; and coupling a second multiplexing block tothe second and third voltage inputs, and coupling a second multiplexingblock output to the second source block, wherein the first voltageoutput and the second source node are configured to be coupled to asecond end of the second plurality of light emitting diodes coupled inseries.
 17. A method of operating a smartphone backlight comprising themethod of claim 15, wherein the backlight comprises the first pluralityof light emitting diodes.
 18. A method of operating a televisionbacklight comprising the method of claim 15, wherein the backlightcomprises the first plurality of light emitting diodes.
 19. A systemcomprising: a backlight comprising a first plurality of light emittingdiodes coupled in series; a first voltage input, second voltage input,and third voltage input; a first voltage generator circuit, coupled tothe first voltage input and generating a first voltage output, whereinthe first voltage output is at a higher voltage level than the firstvoltage input; a first source block comprising a first source node,coupled to a first end of the first plurality of light emitting diodes;and a first multiplexing block coupled to the second and third voltageinputs, and a first multiplexing block output is coupled to the firstsource block, wherein the first voltage output and the first source nodeare configured to be coupled to a second end of the first plurality oflight emitting diodes.
 20. The system of claim 19 wherein the backlightcomprises a second plurality of light emitting diodes coupled in series,and the system comprises: a second source block comprising a secondsource node, coupled to a first end of a second plurality of lightemitting diodes; and a second multiplexing block coupled to the secondand third voltage inputs, and a second multiplexing block output iscoupled to the second source block, wherein the first voltage output andthe second source node are configured to be coupled to a second end ofthe second plurality of light emitting diodes.